Process for producing hydrogen

ABSTRACT

A process for producing hydrogen is provided. The process comprises the introduction of reactants into a reactor with a steam reforming section containing a steam reforming catalyst to form a hydrogen-containing product. The process is driven by heat generated in a combustion section containing an oxidation catalyst, which comprises a noble metal and boron nitride. According to the process of the subject invention, the first combustion reaction can rapidly generate heat and is advantageous for conducting steam reforming reactions.

RELATED APPLICATION

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 10/761,789, filed 21 Jan. 2004, whichis herein incorporated by reference.

TECHNICAL FIELD

The subject invention relates to a process for producing hydrogen. Morespecifically, the invention relates to a process for catalyticallyquick-driving hydrogen production with an integrated catalyzedoxidation.

BACKGROUND OF THE INVENTION

Purified hydrogen is an important fuel source for many energy conversiondevices. For instance, fuel cells normally require hydrogen with anextremely high purity and oxygen (or air) as the fuel to generateelectricity. A widely known process for providing hydrogen is the steamreforming process. Particularly, the steam reforming process comprisesreacting steam with a fuel such as an alcohol (e.g., methanol orethanol) or a hydrocarbon (e.g., methane, gasoline, or hexane) over asteam reforming catalyst to form the main product hydrogen and otherby-products (e.g., CO and CO₂). Since the steam reforming reaction is anendothermic reaction, it requires a substantial amount of heat from anexternal heating system to maintain the temperature of the steamreforming system. Moreover, an additional purification facility is alsonecessary to purify the product formed in the steam reforming reactionto attain the desired purity of hydrogen, typically at least 95% such as95% to 99.995%. Obviously, the external facilities for the steamreforming system, such as heaters and purifying devices, occupy a largershare of the capital investment and plant space.

Since highly-purified hydrogen is desirable in the industry, numerousstudies have been conducted to find an economic and simple way toefficiently purify hydrogen from steam reforming reactions. Membraneseparators have been proposed to harvest purified hydrogen from thesteam reforming process. Additionally, to reduce the space of thereaction system, the combination of the membrane separator with thesteam reformer in a single device, such as a membrane steam reformingreactor, has also been proposed. For example, U.S. Pat. No. 5,861,137discloses a steam reformer with internal hydrogen purification, whichcomprises a tubular hydrogen-permeable and hydrogen selective membranetherein.

Traditional processes normally use external flame in the endothermicsteam reforming reaction mentioned above; however, the complicatedcontrol and the heat transfer efficiency of such system is not alwaysdesirable. It is believed that a rapid and stable supply of heat iscritical in maintaining the desired temperature of the steam reformingzone and thus the reaction rate therein. If not, the transfer of heat tothe reaction zone fails, causing the reaction temperature andconversation rate of hydrogen to drop. As a result, developments havebeen focused on in-situ heating via conventional combustion of fueland/or spent gases from the reformer to provide the heat required forthe endothermic steam reforming reaction. For example, U.S. Pat. No.6,821,502 B2 provides a membrane steam reforming reactor using flamelessdistributed combustion for generating heat. The flameless burning can beprovided by injecting a fuel and a preheated air stream to the reactorfor automatic ignition. Obviously, said technical means needs to preheatthe air with an additional heater prior to feeding it into thecombustion zone. U.S. Pat. No. 5,861,137 discloses a small burner thatis provided to burn the fuel or vent product gases to provide the neededthermal energy. Such manner, however, produces dangerous open flame andpolluting products, e.g., nitrogen oxide. U.S. Pat. No. 6,585,785 B1teaches a fuel processor apparatus comprising a catalyst tubular reactorwhich is heated using an infrared radiant burner to provide theendothermic heat of the reaction needed to reform a mixture ofhydrocarbon and steam for the production of hydrogen. Nonetheless,according to the teachings of U.S. Pat. No. 6,585,785 B1, to provide aneven distribution of thermal energy or temperature in the reactorchamber, complicated facility or device such as forced circulation ofhot air is required.

Apparently, the above mentioned developments concerning the heat supplyto the steam reforming system still have some shortcomings, such as theuse of additional heaters and complicated devices, naked flames, andpolluting products. The subject invention provides a process forproducing hydrogen with high purity (99.99%) in a simple and economicalway. By using the process of the subject invention, the heat generatedfrom a catalytic combustion section can rapidly increase the temperatureof a steam reforming section to a sufficient level to initiate anendothermic steam reforming reaction carried out therein in a very shorttime and to maintain the reaction temperature.

SUMMARY OF THE INVENTION

The objective of the subject invention is to provide a process forproducing hydrogen comprising a step of conducting a steam reformingreaction of reactants. The steam reforming reaction is driven by a heatgenerated from a first combustion reaction, and the first combustionreaction is catalyzed by a supported oxidation catalyst comprising anoble metal and boron nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a reactor module for implementing theprocess of the subject invention.

FIG. 2 is a schematic view showing an assembly of reactor modules forimplementing the process of the subject invention.

FIG. 3 is a schematic view showing a reactor for implementing theprocess of the subject invention.

FIG. 4 is a schematic view showing another reactor for implementing theprocess of the subject invention.

FIG. 5 is a temperature profile showing a first combustion reaction ofmethanol using various catalysts and oxygen/methanol ratios withWHSV=3.2 to start from room temperature, wherein T₁ represents thetemperature at the peak and T₂ represents the temperature at the steady.

FIG. 6 and FIG. 7 show the temperature distributions of the combustionsection and the membrane tube section at different conditions of WHSVand air/MeOH ratio exemplified in EXAMPLE 11.

FIG. 8 shows the temperature variations of the steam reforming sectionof the reactor exemplified in EXAMPLE 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the process of the subject invention, the production ofhydrogen from a steam reforming reaction of reactants is driven by afirst combustion reaction. The first combustion reaction is normally anoxidation of a first fuel over an oxidation catalyst comprising a noblemetal and boron nitride. In particular, the first combustion reactiongenerates heat to allow a section for conducting the steam reformingreaction, i.e., a steam reforming section, to reach a desiredtemperature to initiate and maintain the steam reforming reactioncarried out therein. Optionally, after the steam reforming reaction isinitiated, a portion of a hydrogen-containing product obtained therefromis directed back to the process system as at least a part of the firstfuel for the first combustion reaction, so as to continuously provideheat for maintaining steam reforming section at the desired temperature.Accordingly, the steam reforming reaction can be continuously driven bythe heat generated from the first combustion reaction of thehydrogen-containing product.

The first fuel may comprise a hydrogen-containing gas (such as thehydrogen-containing product obtained from the steam reforming reaction),one or more alcohols (such as C₁₋₄ alcohols), one or more hydrocarbons(such as C₁₋₆ alkanes), and combinations thereof. Specific examples ofthe first fuel include methanol, ethanol, propanol, isopropanol,butanol, methane, ethane, propane, butane, pentane, hexane, gasoline,liquefied petroleum gas, and combinations thereof, and methanol andhexane are preferred. Moreover, according to the subject invention, thefirst combustion reaction is normally carried out at a molar ratio of O₂(from such as air) to C (from the first fuel) ranging from about 1.0 toabout 4.0.

Any noble metal suitable for oxidation can be used in the oxidationcatalyst for the process of the subject invention. Generally, the noblemetal is selected from a group consisting of Pt, Pd, Rh, Ru, and acombination thereof. It is preferred that the noble metal is Pt. Inaddition to the noble metal, the oxidation catalyst used in the subjectinvention comprises boron nitride. Moreover, in application, theoxidization catalyst comprising the noble metal and boron nitride isnormally carried by a support. For example, in one embodiment of thesupported oxidization catalyst used in the subject invention, the noblemetal is dispersed on a horizontal boron nitride layer over a support.The material of the support should be inert and thermally-stable and thesupport is in a porous format. The material of the support can be, butis not limited to, alumina, titania, zirconia, silica, or a combinationthereof. Preferably, the support is consisting essentially of a materialselected from a group consisting of alumina, titania, zirconia, silica,and a combination thereof. A preferred embodiment of the supportmaterial is alumina because of its excellent thermal resistance.Moreover, commercial products such as DASH 220 (NE Chemtec, Inc. Japan)and N220 (Süd Chemie Catalysts, Japan, Inc.) can be used as the support.Generally, based the total weight of the oxidation catalyst system(including the noble metal, boron nitride, and support), the amount ofthe noble metal is from about 0.05 to about 1.0 wt %, and preferablyfrom about 0.1 to about 0.5 wt % and most preferably from about 0.15 toabout 0.25 wt %; and the amount of boron nitride is about 1 to about 20wt %, preferably about 2 to about 10 wt %, and most preferably about 4to about 6 wt %.

The oxidation catalyst can be prepared by any suitable methods. A methodfor preparing the oxidation catalyst is illustrated as followed. A noblemetal salt (e.g., H₂PtCl₆) and boron nitride are first dissolved in asuitable solvent, such as a mixture of methanol and dimethyl formamide(DMF), and then the resulting mixture is stirred for a while to obtain aslurry. Next, the slurry is coated on a support (made of such as Al₂O₃).The coated support is dried and then sintered so as to obtain asupported oxidation catalyst useful in the subject invention.

The boron nitride in the oxidation catalyst serves two functions.Because the oxidization reaction will produce not only heat but alsowater, which is adverse to the catalyst system and will decline thecatalyst efficiency, the hydrophobic character of boron nitride canprevent the generated water from chemisorbing on the active catalystsites too long and facilitate turning over of the sites for new run ofreaction. Additionally, since the thermal conductivity of boron nitrideis high, this helps a rapid transfer of exothermic reaction heat awayfrom the active catalyst center which avoids the formation ofdetrimental hot spots on the catalyst, and also allows an evendispersion of heat in the combustion section for more effective heatsupply to the steam reforming section. This is particularly appreciatedin the reactor scale up design.

In addition, the operation of the subject invention is relatively safebecause the first combustion reaction supplies flameless heat. In otherwords, no dangerous open flames or harmful gases will be produced duringthe steam reforming process. Furthermore, because the steam reformingreaction is an endothermic reaction, a rapid and stable supply of heatis critical to the steam reforming reaction. Using the unique oxidationcatalyst of the subject invention (i.e., comprising the noble metal andboron nitride), the generated heat can be evenly and directlytransferred to the steam reforming section so as to avoid thecomplicated arrangement of forced air circulation for achieving an evendistribution of reaction temperature in the steam reforming section.Consequently, the process of the subject invention can be carried outsmoothly as a result of a stable and effective heat supply from theunique oxidation catalyst.

As mentioned above, the subject invention utilizes the oxidation of thefirst fuel (such as methanol) to generate heat for heating the steamreforming section to a desired temperature, i.e., the reactiontemperature. The inventors also found that prior to the starting of thesteam reforming reaction, a second combustion reaction can be carriedout in the steam reforming section until the steam reforming sectionreaches the desired temperature. In this way, the time required forattaining the steam reforming temperature can be extensively shorten. Inparticular, the second combustion reaction involves an oxidation of asecond fuel (e.g., methanol), which can be identical to or differentfrom the first fuel. Preferably, the first combustion reaction and thesecond combustion reaction are started simultaneously. For example, incomparison with merely utilizing the heat from the oxidation of methanolin the combustion section, the utilization of the heat from theoxidation of methanol in the combustion section as well as that in thesteam reforming section can cut the time that the steam reformingsection reaches the desired temperature (i.e., the temperature of steamreforming reaction) much sooner as much as 50% of initiation time can besaved.

In the steam reforming reaction, the relevant technical contents arewell known in the art. Any materials that can be converted into hydrogenin a steam reforming reaction can be used in the subject invention asthe reactants. Normally, the reactants comprise water as well as one ormore alcohols, one or more hydrocarbons, or combinations thereof. Forexample, the alcohol can be, but is not limited to, methanol, ethanol,propanol, isopropanol, ethylene glycol, glycerol, or a combinationthereof, and the hydrocarbon can be, but not limited to, methane,hexane, gasoline, liquefied petroleum gas (LPG), naphtha oil, dieseloil, or a combination thereof. Preferably, the reactants comprise waterand methanol, hexane, or a combination of methanol and hexane.

To smooth out the steam reforming reaction, the reactants are normallypreheated to a temperature slightly higher than the temperature of thesteam reforming reaction, before being introduced into the steamreforming section. According to the process of subject invention, thereactants can be preheated by the heat generated from the firstcombustion reaction to effectively use the heat in the reaction system.If desired, the reactants can also be preheated using an external heateras described in the prior art, and then fed into the steam reformingsection.

In addition to the heat supply, the steam reforming reaction alsorequires a steam reforming catalyst for lowering the activation energyof the steam reforming reaction to convert the reactants into hydrogen.The steam reforming catalyst is normally selected depending on thespecies of the reactants to be converted into hydrogen. Typical steamreforming catalysts that can be used in the subject invention include,but are not limited to, transition metals. Optionally, the steamreforming catalyst can be used in combination with a group IA metal suchas potassium (K). It is noted that the use of the group IA metal reducethe coking of the catalyst. For example, the steam reforming catalystused in the subject invention can comprise Cu, Zn, Pd, Re, Ni, or acombination thereof. Particularly, in steam reforming of methanol orglycerol, a combination of Cu and Zn can be used as the catalyst. On theother hand, in steam reforming of hexane, a combination of K and Ni canbe used to catalyze the reaction.

Similar to the oxidation catalyst for the first combustion reaction ofthe subject invention, it is often desirable that the steam reformingcatalyst is carried by a support, which is normally an inert compound.Suitable support for the steam reforming catalyst normally comprises oneor more of elements of Group III and IV of the Periodic Table, forexample, oxides or carbides of Al, Si, Ti, and Zr. A preferredembodiment of the support for the steam reforming catalyst is alumina.The method for producing a supported steam reforming catalyst is wellknown by persons skilled in the art, such as the sol gel technique orimpregnation, and can be referred to “Production and thermalpretreatment of supported catalysts,” written by J. W. Geus (seePreparation of Catalysts III, ed. G. Poncelet, P. Grange and P. A.Jacobs, Elsevier, Amsterdam, 1983, 1-34). For example, the supportedsteam reforming catalysts such as CuOZnO/Al₂O₃, PdOCuOZnO/Al₂O₃, andK₂ONiO/Al₂O₃ can be used in the subject invention.

The temperature of steam reforming reaction varies with many factorsincluding the species of the reactants, the scale and module of thereactor for implementing the steam reforming process, and especially,the species of the steam reforming catalyst. For example, in the case ofsteam reforming of an alcohol (such as methanol, isopropanol, orglycerol), the steam reforming catalyst used typically comprises Cu andZn, and the temperature should not go over about 330° C. to prevent thesintering and coking of the steam reforming catalyst. Hence, thetemperature of steam reforming of an alcohol should stay within therange of about 200° C. to about 330° C., preferably, about 280° C. toabout 300° C. On the other hand, for steam reforming of an alkane (e.g.,hexane, methane, or gasoline), the reaction is generally carried out ata temperature of about 700° C. to about 900° C.

The steam reforming reaction of the subject invention provides ahydrogen-containing product. In industrial applications, such as in fuelcells, the hydrogen-containing product always needs to be furtherpurified. As a result, the process of the subject invention preferablyfurther comprises a step of purifying the hydrogen-containing productobtained from the steam reforming reaction to produce purified hydrogenand leave a spent product. Any proper purifying methods, such ascatalytic adsorption, cryogenic cooling, pressure swinging adsorption,or polymer membrane, can be used to conduct the purification.

In the case of using a purifying step, a portion of the spent productobtained from the purifying step can be directed back to the combustionsection as at least a part of the first fuel for the first combustionreaction, to continuously provide heat for maintaining steam reformingsection at a desired temperature. Accordingly, the steam reformingreaction can be continuously driven by the heat generated from the firstcombustion reaction of the spent product.

One preferred embodiment of the process of the subject invention is toutilize at least one palladium membrane tube to purify the product ofthe steam reforming reaction. The palladium membrane tube can be formedby depositing a palladium-containing membrane with a thickness of about3 μm to about 50 μm on a porous support. The palladium-containingmembrane is normally made from one of the following materials:palladium, a palladium-silver alloy, and a palladium-copper alloy. Theporous support can be made of such as ceramic material or stainlesssteel. Stainless steel is preferred because of its cost effectivenessand convenience in the fabrication of a reactor.

The palladium-containing membrane can be deposited on the porous supportusing an electroplating method, an electro-less plating method, asputtering method, or a cold-rolled method. Many prior art references,such as TW 1232888, U.S. Pat. No. 6,152,987, JP 2002-119834, and JP2002-153740, already describe the technology for depositing apalladium-containing membrane on a porous support and their contents areincorporated hereinto for reference.

As exemplified in Examples 3 and 4 below, in one embodiment of thepalladium membrane tube suitable for the subject invention, the outsidediameter of the tube is 9.525 mm, while the length of the tube is 150mm. Furthermore, the palladium membrane tube has one sealed end, whichis arranged upstream to the flowing path to speed up the flow ofhydrogen permeating from the sealed end to the open end. The crudehydrogen, with a 60-75% purity, from the steam reforming reactionpermeates through the palladium membrane tubes to yield hydrogen with apurity greater than 99%. The high purity of hydrogen is directly derivedin the membrane tube side without any additional purificationfacilities.

Normally, the temperature of conducting the purification with the use ofone or more membrane tubes is not higher than about 490° C., such asabout 25° C. to about 490° C. Preferably, the purification is carriedout at a temperature ranging from about 200° C. to about 380° C. Theheat for maintaining the purification temperature can also be providedby the combustion section.

In the purification step, highly pure hydrogen is separated from thespent product. The spent product primarily contains CO and CO₂, and alsocontains H₂. As mentioned above, a portion of the spent product can bedirected back to the combustion section to generate heat forcontinuously supplying heat to the endothermic steam reforming reaction.The spent product can also be used in many other applications, such asheating water. The highly pure hydrogen obtained may contain fewundesired carbon-containing compounds such as CO and CO₂, which areunfavorable to many energy conversion devices, especially fuel cells,and will reduce their efficiency. Accordingly, it is preferred tofurther treat the highly pure hydrogen with a converter to convert theundesired carbon-containing compounds into an alkane, such as methane.

It is unexpectedly observed that the structure of the reactor willinfluence the temperature for conducting the steam reforming reaction.Particularly, it is observed that when the hydrogen purification isachieved by using palladium membrane tubes and the tubes are configuredin the steam reforming section, the temperature necessary to conduct thesteam reforming reaction can be reduced. For example, when gasoline isused as the reactant for the steam reforming reaction, the steamreforming temperature can be reduced from at least about 700° C. (e.g.,from about 700° C. to about 900° C.) to less than about 650° C. (e.g.,from about 500° C. to about 650° C.). Without limited by theory, it isbelieved that the reduction of hydrogen in the steam reforming sectiondue to the hydrogen-permeable palladium membrane can break the limits ofthermodynamic control on the conversion level and attain the sameconversion of hydrogen at a lower temperature.

The apparatus for carrying out the process of the subject inventionmainly comprises three sections, i.e., the steam reforming section, thehydrogen purification section, and the combustion section. It should bementioned that the three sections can be arbitrarily arranged accordingto different needs of scales and temperature requirements given that thecombustion section can be positioned inside, outside, or between theother two sections.

For implementing the process of the subject invention, a 3-in-1 reactormodule can be used. In brief, the reactor used combines three sections,namely, a steam reforming section for producing a hydrogen-containingproduct, a membrane tube section containing at least one palladiummembrane tube for purifying the hydrogen-containing product, and acombustion section for providing the heat required for driving the steamreforming reaction. For example, the membrane tube section can bearranged within the steam reforming section. In other words, thepalladium membrane tube and the steam reforming section are positionedin the same compartment. Alternatively, the membrane tube section andthe steam reforming section can be arranged in separate compartments ofthe reactor. As for the combustion section, as mentioned above, it canbe configured inside, outside, or between the membrane tube section andthe steam reforming section as required.

One reactor module for implementing the process of the subject inventionis shown in FIG. 1. Reactor module 1 comprises a reactor 15 with a shell11 that has an inlet 12, an outlet 13 and a vent 14; a flowing path 17extending from the inlet 12 to outlet 14; and several palladium membranetubes 16. The palladium membranes deposited on each of the tubes 16 areused for purifying hydrogen, wherein each tube 16 has one sealed endlocated upstream to the flowing path 17. The reactor module 1 furtherincludes a combustion section 18 for heating the reactor 15. The inlet12 is configured to receive reactants composed of steam and a fuel, suchas gasoline, after the reactants are pumped from the feed tank 121 andis properly heated to the desirable reaction temperature by the heatgenerated in the combustion section 18. The outlet 13 is configured todischarge pure hydrogen, while the vent 14 is configured to discharge aspent product, including H₂, CO and CO₂. The spent product dischargedfrom the vent 14 is passed through a pressure reducer 191 and forwardedinto the combustion section 18 through a connection 19 for combustion. Aproper amount of air is pumped first through a fuel reservoir 182 and acheck valve 183 and then into the connection 19 to mix with the spentproduct. The gases from the combustion section 18 are further ventedthrough an outlet 181 for discharge as waste gases or heat exchangedwith the feed stream. Moreover, reactor module 1 can further include aheat conductive perforated metal plate 151 welded to the wall of thereactor 15. The heat conductive perforated metal plate 151 facilitatesheat transfer from the warmer reactor wall to the steam reformingcatalyst zone for the endothermic reaction. In the case of steamreforming of hexane or gasoline, the reactor module 1 is suitable.

It is observed that hydrogen flux through the palladium membrane isdrastically decreased when the hydrogen concentration is low. This meansthat the palladium membrane tube is very inefficient in the low hydrogenconcentration region. This surprising discovery practically limits theuse of long length membrane tubes during hydrogen production on a largescale. Accordingly, another reactor module with a short palladiummembrane tube can be used to conduct the process of the subjectinvention. Preferably, the length of the palladium membrane tube isabout 3 cm to about 120 cm. Moreover, in order to avoid using a longtube, an assembly 2 of the reactor modules is useful as shown in FIG. 2.

Referring to FIG. 2, an assembly 2 includes two reactor sections 28 and29, both with an extended common shell 21, an inlet 22, two vents 24 and25, and two outlets 26 and 27. The two reactor sections 28 and 29 areassembled to share the common inlet side and have flowing pathsextending from the inlet to the outlet opposite in direction to thereactor sections 28 and 29, respectively. Each reactor section, 28 or 29has a plurality of palladium membrane tubes 30. The palladium membranetube 30 is formed by depositing a palladium membrane on the poroussupport for purifying hydrogen, wherein each palladium membrane tube 30has one sealed end located upstream to the flowing path. Assembly 2further includes a combustion section 31 for heating the reactorsections 28 and 29. The inlet 22 is configured to receive reactants. Thereactants can comprise ethanol, methanol, isopropanol, methane, hexane,gasoline, LPG, glycerol, or a combination thereof. The outlets 24 and 25are configured to discharge pure hydrogen, and the vents 26 and 27 areconfigured to discharge spent products including H₂, CO, and CO₂. Thespent products discharged from the vents 26 and 27 are introduced intothe combustion section 31 through a connector (not shown) forcombustion. The waste gases are discharged from the combustion section31 via a vent (not shown). Moreover, the assembly 2 further includes aheat conductive perforated metal plate 23 welded into the reactor wallin each reactor section. The heat conductive perforated metal plate 23facilitates heat transfer from the warmer reactor wall to the steamreforming catalyst zone in the endothermic reaction.

In addition to the above reactors having the configuration that thepalladium membrane tubes are arranged inside the steam reformingsection, other reactor types can be used in the subject invention. Oneembodiment of the reactors is depicted in FIG. 3. As shown in FIG. 3, areactor 300 comprises a membrane tube section 340 with at least onepalladium membrane tube 350 located in the central part of the reactor300, a steam reforming section 310 located in the peripheral part of thereactor 300, and a combustion section 330 located between the membranetube section 340 and steam reforming section 310. A first fuel, such asmethanol, and air are first introduced into the combustion section 330via a line 331 for combustion. The combustion section 330 is filled withthe oxidation catalysts. In this aspect, the first fuel and air can beintroduced into the combustion section 330 using different inlets. Forexample, the first fuel can be pumped into the combustion section 330 atthe bottom and the middle of the reactor 300 and air can be fed at thebottom of the reactor 300. Then, the first fuel is reacted with theoxygen subject in the air over the oxidation catalyst in the combustionsection 330 to rapidly generate heat. Then, reactants comprising a fueland water are pumped into a preheating coil 320 located in thecombustion section 330 via a line 311 that is heated to a predeterminedtemperature of about 20° C. to about 50° C. higher than the temperatureof the steam reforming reaction. The space velocity of the reactantsdepends on many factors, such as the size of the reactor and thecomponents of reactants. Generally, the reactants are introduced intothe reactor 300 with a space velocity of about 0.9 to about 5.0 hr⁻¹,preferably about 2.0 to about 4.0 hr⁻¹. Then, the pre-heated reactantsare introduced into the steam reforming section 310 via a line 312.Meanwhile, the steam reforming reaction will be quickly driven by theheat generated in the combustion section 330 and is conducted smoothlydue to the stable heat supply.

After, the product formed in the steam reforming section 310 is fed intoa membrane tube section 340 via a line 313. The product includeshydrogen, un-reacted reactants, and by-products. In the membrane tubesection 340, the hydrogen in the product will permeate the palladiummembranes on the palladium membrane tubes 350, and flow in and exit fromthe palladium membrane tubes 350. Purified hydrogen can then be obtainedvia a line 314. The palladium membrane tubes 350 have one sealed end,which is arranged upstream to the flowing path to speed up the flow ofhydrogen from the sealed end to the open end. Also, since the presenceof CO and CO₂ in a fuel cell is undesirable, the purified hydrogen fromthe palladium membrane tubes 350 is further treated with a methanizer360 to convert CO and CO₂ in the purified hydrogen to CH₄. The heat formaintaining the methanizer 360 at a desired temperature can also beprovided by the combustion section 330. By using the reactor 300 forimplementing the process of the subject invention, the purity of theresulting purified hydrogen is above 99.98%.

The spent product, which does not permeate through the palladiummembrane, exits from the membrane tube section 340 via a line 315. Someof the spent product can be directed back into the combustion section330 for conducting catalytic oxidization so as to generate heat. Then,the spent gas which is generated in the combustion section 330 exitsfrom the reactor 300 via a line 332. The reactor 300 is particularlysuitable for use in steam reforming of methanol because the steamreforming section 310 deposited in the peripheral part can be maintainedat a lower temperature (e.g., about 280° C. to about 300° C.).

FIG. 4 shows another reactor for use in conducting the process of thesubject invention. The reactor 400 comprises a membrane tube section 440deposited in the peripheral part of reaction 400 and a steam reformingsection 410 positioned in a combustion section 430. The steam reformingsection 410 is in tubular form. As described in FIG. 3, the first fueland air for combustion are introduced into the combustion section 430via lines 433 and 431, respectively. Then, the reactants are first fedinto the preheating zone 420 via a line 411 and then into the steamreforming section 410. The product formed in steam reforming section isintroduced into the membrane tube section 440 with the palladiummembrane tubes (not depicted) via a line 412 for hydrogen purification.After hydrogen purification, the purified hydrogen is obtained via aline 414, while the spent product is directed back into the combustionsection 430 via a line 415. Waste gases in the combustion section 430exit the reactor 400 via a line 432. The reactor 400 further includes aheat conductive perforated metal plate 460 welded into the reactor wallto facilitate the uniform distribution of the generated heat.

EXAMPLES Example 1 Hydrogen Permeation of a H₂—CH₄ Mixture

The hydrogen mixture with different concentrations of hydrogen, i.e.,99.995%, 80%, 75%, and 66%, were used to study hydrogen permeationthrough the palladium membrane tube at 330° C. under a pressure of 5, 6,7 and 8 bar at the shell side. The resultant hydrogen flux through thepalladium membrane is shown in Table 1. The permeability was calculatedin units of M3/M2-hr-bar1/2. The experiment was carried out in astainless steel tubular reactor that was 25 mmOD×350 mL (outsidediameter×length) with a palladium membrane tube of 9.525 mmOD×110 mmL.The hydrogen mixture is fed into the shell side of membrane, and thenthe pure hydrogen permeates through the membrane into the interior ofthe membrane tube. The permeation pressure is set by adjusting the backpressure regulator in the spent gas mixture stream before leaving thereactor system. TABLE 1 Hydrogen permeation of a H₂/CH₄ mixture with apalladium membrane Perme- Flux, ability, % H in % M³/M²-hr P1, absol.,bars M³/M²- H₂/CH₄ H₂ purity 6 7 8 hr-bar^(1/2) 99.995 Flux, 18.30 21.2824.00 12.6 M³/M²-hr H₂ purity 99.99999+ 80 Flux, 13.85 16.66 18.96 11.85M³/M²-hr H₂ purity 99.96 75 Flux, 12.71 14.83 16.94 11.45 M³/M²-hr H₂purity 99.92 66 Flux, 10.86 12.96 14.28 10.8 M³/M²-hr H₂ purity 99.92

Example 2 Hydrogen Permeability of a H₂—Y Mixture with Y: N₂, CO₂ andCyclohexanol (CXL)

The palladium membrane tube, which was 9.525 mm×30 mm (outsidediameter×length), was used for the hydrogen permeability test at 310° C.The results are shown in Table 2. The observed drop in hydrogenpermeability was far more than that could be accounted for by thedecrease of the partial pressure of hydrogen. Moreover, the dilution ofhydrogen concentration brought about not only a decrease in hydrogenflux, but also a deterioration of the hydrogen purity via thepermeation. Through the palladium membrane, an industrial grade ofhydrogen with 99.995% purity can be purified into an electronic gradewith 99.9999+purity. The purity was decreased to 99.9999% and 99.99%when the hydrogen concentration was decreased to 75% and 50%,respectively, as the Y was CXL. TABLE 2 Hydrogen permeability in a mixedfeed of H₂/Y (Y = CO₂, CXL, or N₂)^([a]) Perme- Flux, ability, % H in %cc/min P1, absol., bars M³/M²- H₂/Y Y H₂ purity 3 4 5 hr-bar^(1/2)99.995 Flux, 87.3 122 152 8.66 cc/min H₂ purity 99.9999+ 75 CXL^([b])Flux, 81 113 140 9.30 cc/min H₂ purity 99.9999 50 CXL Flux, 24 35 475.08 cc/min H₂ purity 99.992 99.994 99.996 50 N₂ Flux, 20 34 42 4.48cc/min H₂ purity >99.9^([c]) 50 CO₂ Flux, 13 23 32 3.60 cc/min H₂ purity99.94 99.95 99.96^([a])The permeability test was conducted at 310° C. with a Pd-membraneof 9.575 mm × 30 mm (outside diameter × length). The products wereanalyzed with a GC-FID capable of detecting an impurity up to 1 ppm ofCOx (CO and CO₂) and other organic compounds.^([b])CXL = cyclohexanol^([c])Analyzed with a TCD that has a nitrogen sensitivity >0.5% in thepermeation.

Example 3 Direction of Hydrogen Permeation in a Pd-Membrane Tube withRespect to the Membrane Sealing

A palladium membrane tube (9.525 mmOD×150 mL) with one sealed end wasinserted into a tubular reactor (25.4 mmID×450 mL) via two modes ofconnections, [A] and [B]. In the [A]-mode, the sealed end of themembrane tube was arranged upstream to the hydrogen flow, while thehydrogen flowed into both the shell-side and the tube-side co-currently.In the [B]-mode, the sealed end of the membrane tube was arrangeddownstream to the hydrogen flow. The hydrogen flowed into both theshell-side and tube-side. When the permeation pressure in the shell sidewas set at 3 bar, the hydrogen flux in the tube side in the [A]-mode was210 cc/min, while the corresponding hydrogen flux in the [B]-mode was192 cc/min.

Example 4 Direction of Hydrogen Permeation in the Pd/Ag-Membrane Tubewith Respect to the Membrane Sealing

A similar experiment to Example 3 was further tested with a 67/33 weightratio of a Pd/Ag alloy membrane tube that was 25 μm thick. The membranetube had a similar porous support (9.525 mmOD×150 mL) with the sealedend inserted into a tubular reactor of 25.4 mmID×450 mL via two modes ofconnections, [A] and [B] as described above. In the [A]-mode, the sealedend of the membrane tube was arranged upstream to the hydrogen flow,while the hydrogen flowed into the shell-side and tube-sideco-currently. In the [B]-mode, the sealed end of the membrane tube wasarranged downstream of the hydrogen flow, while the hydrogen flowed intothe shell-side and tube-side. When the permeation pressure in the shellside was set at 3, 4, and 6 bar, the hydrogen flux in the tube side in[A]-mode was 95, 136, and 189 cc/min, respectively. The correspondinghydrogen flux in the [B]-mode was 80, 112, and 170 cc/min, respectively.

Example 5 Preparation of an Oxidation Catalyst on a Supporting Material

Five grams (5 g) of H₂PtCl₆ and 50 grams of boron nitride were dissolvedin a solvent comprising 800 ml of methanol and 200 ml of dimethylformamide, and then stirred to obtain a slurry. The slurry was thencoated on alumina (948 g), and the coated alumina was dried at atemperature of 100° C. to remove the solvent. Next, the dried aluminawas sintered in an oven at a temperature of 450° C. with an air flow of5 L/min for 8 hours to obtain the oxidation catalyst on alumina.

Example 6 Cold Start Heating with a PBN Oxidation Catalyst

Six grams (6 g) of Pt/BN/γ-Al₂O₃ was used as the oxidation catalyst andwas placed in a stainless steel tube with a ½-inch OD (outsidediameter). The whole tube was insulated with mineral wool. Two sets oftemperatures were measured by thermocouples as T_(a) and T_(b). T_(a)indicated the temperature at the top of catalyst bed, while T_(b)indicated the temperature outside of the tube and adjacent to the top ofcatalyst. An appropriate amount of methanol was pumped into thestainless steel tube at a desired space velocity (WHSV, hr⁻¹) and airwas introduced to provide a molar ratio of O₂/Methanol close to 1.65 or1.80 (corresponding to 10% and 20% excess of theoretical demand). As aresult, the reaction temperatures indicated as T_(a) and T_(b) roserapidly from room temperature to about 800° C. and then stabilized to alower temperature of about 400° C. to 450° C. when a molar ratio ofO₂/Methanol close to 1.65 or 1.80 was introduced at an appropriate spacevelocity of 2 hr⁻¹ to 4 hr⁻¹ (WHSV). In addition, the oxidationcatalysts in the catalytic combustion section were either Pt/BN—N-220and Pt/BN-Dash-220. The heating effect and cold start capability ofPt/BN—N-220 and Pt/BN-Dash-220 are shown in FIG. 5, wherein T₁represents the temperature at the peak and T₂ represents the steadytemperature.

Example 7 High Temperature from the Cold Started Catalytic Combustion ofHexane with Pt/BN—N-220

Six grams (6 g) of PtBN/N-220 were used as the oxidation catalyst, andwas placed in the combustion section according to the subject invention.N-hexane, as the first fuel, was pumped onto the oxidation catalyst at avelocity of 1.66 gm/min. Then, airflow was introduced at a velocity of2.35 L/min to give an O₂/hexane ratio close to 10.45 (10% excess oftheoretical demand). The temperature indicated as T₃ in the catalystzone rose to 630° C. in 4 min, and then to 970° C. in another 5 min. Thetemperature indicated as T₄ was maintained between 980° C. to 960° C.for the next 110 min until the reaction was terminated. Apparently, heatfrom the combustion of hexane for maintaining the steady temperature, T₄was much more than that of methanol. On the other hand, it was easierfor methanol to initiate the oxidation reaction. Initially, T₁ rosefaster and higher than T₃.

Example 8 Cold Start of the Methanol Steam Reforming Reaction UsingCatalytic Combustion of Aqueous Methanol

Twelve grams (12 g) of Pt/BN-γ-Al₂O₃, as the oxidation catalyst, wasplaced into the combustion section of the subject invention. 120 g ofCuO-ZnOAl₂O₃, as the steam reforming catalyst, was placed in thereactor. The reactor was then wrapped with a thick layer of mineral woolfor insulation. Both the inner catalyst zone and annular catalyst zonewere independently connected with metering pumps for delivery of amethanol-water mixture for the reforming and combustion fuel,respectively. Initially, methanol, as the fuel, at a feeding rate of 7.5mmol/min. with WHSV=1.2, was introduced into the combustion reactor atroom temperature to set the oxidation reaction by reacting with air at aO₂/CH₃OH ratio close to 1.65 in 12 minutes. The temperature, T_(OX), ofthe oxidation catalyst in the catalytic combustion section rose to 560°C. and almost simultaneously the temperature, T_(SR), in the reactorreached 380-390° C. The endothermic steam reforming reaction of methanolwas then started up by introducing liquid methanol (15 mmol/min)together with water (18 mmol/min) into the reforming catalyst bed. Thereaction temperature, T_(SR), dropped slightly to 350° C. and keptsteady for the next 60 minutes. Hydrogen and carbon oxides were producedfrom the reforming side of the reactor. Thereafter, T_(OX) in thecombustion section dropped to 420-460° C. and T_(SR) in the reformerdecreased to 310° C. The lower temperature reactions were continued foranother 30 minutes while the gaseous products evolved smoothly.

Example 9 Steam Reforming Reaction of Hexane

According to the subject invention, a steam reforming reaction of hexanewas carried out at 500° C. under a pressure of 9 bars and VHSV of 10,000to 30,000 hr⁻¹. Five steam reforming catalysts were tested for theircarbon coking rate, conversion and their selectivity to hydrogenproduct. The characteristics of these catalysts are shown in Table 3 andtheir performance is presented in Table 4 for comparison. As shown inTables 3 and 4, both commercial catalysts had a higher decaying rate bycoking than the three homemade catalysts with a higher surface area. Inaddition, the commercial catalysts also brought about higher a hydrogenpartial pressure and higher selectivity to the hydrogen product. Withregard to the Y-2 catalyst, the use of palladium membrane tube showed amuch higher conversion, hydrogen partial pressure and the selectivity tohydrogen. TABLE 3 Characteristics of catalysts used for the steamreforming of hexane G-56H-1 FCR-4-02 Y1 Y2 Y3 Specific 2.3 3.0 2.0 2.02.0 gravity (kg/l) Surface 27.41 6.73 145.32 133.6 131.44 area (m²/g)Pore volume 0.054 0.014 0.365 0.338 0.32 (c.c./g) Pore size (A) 78.887.6 100.33 101.1 97.3 Support α-Al₂O₃ α-Al₂O₃ γ-Al₂O₃ γ-Al₂O₃ γ-Al₂O₃Content Ni 17.0 12.0 15.0 15.0 17.0 (%) K₂O 0.4 - 0.4 1.0 0.4 MgO — — —— 5.0

TABLE 4 The performance of steam reforming reaction of n-hexane^([a])Av. Coking rate n-Hexane Partial Press Gas composition (vol %) Catalyst(mgC/gCat-hr)^([b]) Conv. (% mol) of H₂ (%) CO CO₂ CH₄ H₂ FCR-4-02 128.348.02 14.93 0.59 22.55 38.69 38.18 G-56-H-1 13.29 65.84 12.15 0.56 21.6450.23 27.57 Y-1 8.23 40.48 21.31 0.15 21.52 17.50 59.49 Y-2 3.20 42.3921.16 0.97 22.35 12.67 64.00 Y-2/Membr 3.20 46.05 31.94 0.02 23.18 9.5168.29 Y-3 7.70 39.69 18.38 1.51 21.92 3.57 72.99^([a])VHSV = 20000 h⁻¹, H₂O/C = 1.5, 9 atm, 500° C.^([b])Coking time of 6 hr under the conditions of [a]

Example 10

A reactor (Green Hydrotec Inc., Model: GHR500LPH100) with a structure asshown in FIG. 3 was used in this example. Methanol was introduced intothe combustion section of the reactor. Upon achieving a temperature of260° C., methanol and water in a molar ratio of 1:1.2 was introducedinto the steam reforming section of the reactor with a feeding rate of10 g/min. The amount of the purified hydrogen from the palladiummembrane tubes was 7.5 L/min. The amount of the spent product(containing H₂ and CO₂) was 10 L/min. After calculations, H₂ recoveryyield was 57.1%.

The spent product (10 L/min) was divided into two streams. One stream(5.2 L/min) was introduced back into the combustion section to generateheat for preheating coil and steam reforming. Another stream (4.8 L/min)was introduced into another oxidation zone to heat 162 g of water from20° C. to 49° C. The thermal energy provided by the stream (4.8 L/min)was 19.6 KJ/min. The total thermal efficiency of the reactor was 78%.

Example 11 Different Values of WHSV and Air/MeOH Ratio on theTemperature Distribution of Combustion Section and Membrane Tube Section

In the case of GHT500LPH100, the space velocity, WHSV and Air/MeOH ratiowere changed to test the optimal temperature distribution of thereactor. Table 5 lists the experimental conditions for this example. Thetemperature distribution of the combustion section and the membrane tubesection are shown in FIGS. 6 and 7. All four conditions can have smoothtemperature distributions without hot spots. The air flow rate wasreduced to cut heat loss by the excess air vent as indicated by EXP1 andEXP3. Furthermore, EXP2 using the most methanol or fuel input withenough oxygen for combustion exhibited the highest temperature profile.EXP4 provides a high enough temperature profile for heat transfer withlower excess air, and is most efficient. The higher WHSV, on the otherhand, provides a higher reactor temperature for faster heat timetherefore, it is useful in bringing up the reaction temperature in theinitial stage. TABLE 5 Air MeOH WHSV EXP No. (L/min) (g/min) (1/hr) O₂/CExcess air 1 60 5 0.375 3.29 119% 2 30 4.16 0.312 1.98 32% 3 30 3.520.264 2.34 56% 4 20 3.52 0.264 1.56 4%

Example 12 Introduction of a First Fuel and Air into the Steam ReformingSection and the Combustion Section

The conditions are as listed below:

For the steam reforming section (SRR):

WHSV (feeding rate of methanol/catalyst weight): 1.54 hr⁻¹

Catalyst weight: 50 g of CuOZnO/Al₂O₃

Feeding rate of air: 6 L/min

For the combustion section (OXD):

WHSV (feeding rate of mathanol/catalyst weight): 3.8 hr⁻¹

Catalyst weight: 20 g of Pt—BN/Al₂O₃

Feeding rate of air: 6 L/min

As shown in FIG. 8, the results show that introducing the methanol andair into the steam reforming section prior to feeding the raw materialof the first fuel and water can shorten the time for heating the steamreforming section to the reaction temperature.

The above disclosure is related to the detailed technical contents andinventive features thereof. People skilled in this field may proceedwith a variety of modifications and replacements based on thedisclosures and suggestions of the invention as described withoutdeparting from the characteristics thereof. Nevertheless, although suchmodifications and replacements are not fully disclosed in the abovedescriptions, they have substantially been covered in the followingclaims as appended.

1. A process for producing hydrogen comprising a step of conducting asteam reforming reaction of reactants, wherein the steam reformingreaction is driven by a heat generated from a first combustion reaction,and the first combustion reaction comprises conducting an oxidation of afirst fuel and is catalyzed by an oxidation catalyst comprising a noblemetal and boron nitride.
 2. The process according to claim 1, whereinthe noble metal is selected from a group consisting of Pt, Pd, Rh, Ru,and a combination thereof.
 3. The process according to claim 1, whereinthe noble metal is Pt.
 4. The process according to claim 1, wherein theoxidation catalyst is carried by a support consisting essentially of amaterial selected from a group consisting of alumina, titania, zirconia,silica, and a combination thereof.
 5. The process according to claim 4,wherein the material is alumina.
 6. The process according to claim 1,wherein the first fuel comprises a hydrogen-containing gas, an alcohol,a hydrocarbon, or a combination thereof.
 7. The process according toclaim 6, wherein the alcohol is selected from a group consisting ofmethanol, ethanol, propanol, isopropanol, butanol, and combinationsthereof, and the hydrocarbon is selected from a group consisting ofmethane, ethane, propane, butane, pentane, hexane, gasoline, liquefiedpetroleum gas (LPG), and combinations thereof.
 8. The process accordingto claim 6, wherein the first fuel comprises a portion of ahydrogen-containing product and the hydrogen-containing product isproduced by the steam reforming reaction.
 9. The process according toclaim 6, wherein the first fuel comprises methanol.
 10. The processaccording to claim 6, further comprising conducting a second combustionreaction in a steam reforming section for conducting the steam reformingreaction until the steam reforming section reaches a desired temperatureprior to the starting of the steam reforming reaction.
 11. The processaccording to claim 10, wherein the second combustion reaction comprisesconducting an oxidization of a second fuel, wherein the second fuel isidentical to or different from the first fuel.
 12. The process accordingto claim 10, wherein the first combustion reaction and the secondcombustion reaction are started simultaneously.
 13. The processaccording to claim 1, wherein the reactants comprise water as well as analcohol, a hydrocarbon, or a combination thereof.
 14. The processaccording to claim 13, wherein the alcohol is selected from a groupconsisting of methanol, ethanol, propanol, isopropanol, ethylene glycol,glycerol, and combinations thereof, and the hydrocarbon is selected froma group consisting of methane, hexane, liquefied petroleum gas (LPG),gasoline, naphtha oil, diesel oil, and combinations thereof.
 15. Theprocess according to claim 13, wherein the reactants comprise water aswell as methanol, hexane, or a combination thereof.
 16. The processaccording to claim 1, wherein the steam reforming reaction is catalyzedby a catalyst comprising Cu, Zn, Pd, Re, Ni, or a combination thereof.17. The process according to claim 16, wherein the steam reformingreaction is catalyzed by a catalyst comprising K as well as Cu, Zn, Pd,Re, Ni, or a combination thereof.
 18. The process according to claim 1,further comprising a step of purifying the product obtained from thesteam reforming reaction to provide hydrogen with a relatively highpurity and a spent product.
 19. The process according to claim 18,wherein the first fuel comprises a portion of the spent product.
 20. Theprocess according to claim 18, wherein the purifying step is conductedwith the use of at least one palladium membrane tube.
 21. The processaccording to claim 20, wherein the palladium membrane tube is formed bydepositing a palladium-containing membrane on a porous support, and theporous support is made of stainless steel or a ceramic material.
 22. Theprocess according to claim 21, wherein the palladium-containing membraneis made of palladium, a palladium-silver alloy or a palladium-copperalloy.
 23. The process according to claim 20, wherein the hydrogenobtained from the purifying step has a purity of at least 99%.
 24. Theprocess according to claim 18, further comprising a converting step toconvert any carbon-containing compounds contained in the hydrogen intoalkane.
 25. The process according to claim 20, the process is conductedin a reactor comprising a steam reforming section, a combustion section,and a membrane tube section, the steam reforming reaction is carried outin the steam reform section, the first combustion reaction is carriedout in the combustion section, and the purifying step is conducted inthe membrane tube section, wherein the steam reforming section isarranged in the peripheral part of the reactor, the membrane tubesection is arranged in the central part of the reactor and comprises atleast one palladium membrane tube, and the combustion section is locatedbetween the membrane tube section and the steam reforming section. 26.The process according to claim 1, wherein the reactants comprise waterand methanol.
 27. The process according to claim 26, wherein the steamreforming reaction is conducted at a molar ratio of methanol/waterranging from about 1.0 to about 1.5.
 28. The process according to claim27, wherein the molar ratio of methanol/water ranges from about 1.05 toabout 1.25.
 29. The process according to claim 26, wherein the steamreforming reaction is conducted at a temperature ranging from about 200°C. to about 330° C.
 30. The process according to claim 29, wherein thesteam reforming reaction is conducted at a temperature ranging fromabout 280° C. to about 300° C.
 31. The process according to claim 20,wherein the purifying step is conducted at a temperature of not higherthan about 490° C.
 32. The process according to claim 31, wherein thepurifying step is conducted at a temperature ranging from about 25° C.to about 490° C.
 33. The process according to claim 31, wherein thepurifying step is conducted at a temperature ranging from about 200° C.to about 380° C.